The present invention relates generally to X-ray spectrometry, and specifically to methods and devices to detect and analyze X-ray microfluorescence.
X-ray microfluorescence analysis is a non-destructive technique known in the art for determining the atomic composition and thickness of thin films. Typically, a focused X-ray beam is directed at a sample, and the X-ray fluorescence induced by the interaction of the X-rays with the sample is detected by a detector located near the sample. The composition and thickness of the irradiated sample are determined from the intensity and energy of the fluorescent X-ray photons.
In xe2x80x9cAnnular-Type Solid State Detector for a Scanning X-Ray Analytical Microscope,xe2x80x9d Review of Scientific Instruments 66(9) (September, 1995) pp. 4544-4546, which is incorporated herein by reference, Shimomura and Nakazawa describe an annular germanium detector located near an irradiated sample which transduces the energy resulting from X-ray fluorescence into a single channel of data.
U.S. Pat. No. 5,937,026, to Satoh, whose disclosure is incorporated herein by reference, describes a microfluorescent X-ray analyzer in which a capillary tube is used to deliver X-ray excitation to a small region of a sample. The capillary passes through a hole in the center of a flat plate solid-state X-ray detector, which is used to detect fluorescent X-rays emitted by the sample. The geometry of the capillary tube and the detector allows fluorescent X-rays from a small excitation region to be detected over a large solid angle.
U.S. Pat. No. 3,256,431, to Fraser, U.S. Pat. No. 3,581,087, to Brinkerhoff and U.S. Pat. No. 5,778,039, to Hossain, whose disclosures are incorporated herein by reference, describe systems for detection and analysis of X-ray fluorescence using multiple detectors. In all of these patents, a sample is excited by an X-ray source, and the multiple detectors are used to detect the X-ray fluorescence in different, respective energy domains. Typically, the energy domains are chosen to correspond to emission bands of different elements in the sample, so that comparative measurements can be made of the relative concentrations of two elements, for example.
U.S. Pat. No. 5,497,008, to Kumakhov, which is incorporated herein by reference, describes analytic instruments using a polycapillary X-ray optic, also known as a Kumakhov lens, for X-ray fluorescence analysis or spectroscopy. The instruments described use a single fluorescence detector.
It is an object of some aspects of the present invention to provide improved apparatus and methods for X-ray microfluorescence analysis.
It is a further object of some aspects of the present invention to provide apparatus and methods for detection and analysis of X-ray microfluorescence associated with very small geometrical features of a sample.
It is yet a further object of some aspects of the present invention to provide apparatus and methods for detection of faults occurring in production of semiconductor devices.
In preferred embodiments of the present invention, an X-ray microfluorescence analyzer comprises an X-ray source which irradiates a small spot on a sample, and a plurality of individual detectors arrayed around the spot, so as to capture X-ray photons emitted from the sample responsive to the X-ray illumination. Preferably, the detectors are arrayed in a generally symmetrical pattern about the spot. A processing unit receives signals from the detectors and processes them to compare the intensity of photon emission captured by the different detectors, and thus to detect variations in the intensity as a function of azimuth about the irradiation beam. These variations are indicative of directional inhomogeneity of the emission from the sample.
The detected azimuthal differences in the intensity of emission in a selected energy range are preferably used to determine properties of microscopic structures in the sample under test. Alternatively or additionally, the differences are monitored in order to accurately align the X-ray source and detectors with such structures. The method of the present invention, wherein multiple detectors are used simultaneously to measure emission in a common energy range at different azimuths, is substantively different from methods of X-ray fluorescence analysis known in the art. Such methods, as described in the Background of the Invention, are generally based on detection at only a single azimuth at any given time. When multiple detectors are used, their purpose is to measure emission in different, respective energy ranges, and directional inhomogeneity of emission is not considered.
In some preferred embodiments of the present invention, the analyzer is used to measure overlay errors between successive layers, such as metallization layers, created on a semiconductor wafer in the course of integrated circuit production. Preferably, a test zone is created on the wafer, in which a pattern in a lower layer, using a first element,is overlaid by a substantially identical pattern in an upper layer, using a second, different element. The first and second elements are typically metal elements, although other types of X-ray detectable elements may also be used. When the layers are in proper registration, the pattern in the upper layer substantially shields the element in the lower layer from X-rays and prevents X-ray photons from the first element from reaching the detectors. When there is a registration error, however, a portion of the pattern in the lower layer is exposed to X-rays, so that photons from the first element can reach the detectors. The processing unit analyzes the intensity and direction of emission of these X-ray photons in order to determine the degree and direction of misregistration between the upper and lower layers.
In other preferred embodiments of the present invention, the analyzer is used to determine the composition and thickness of bumps formed on a surface of the sample. Such bumps typically comprise metal bumps, which are formed on the upper surface of a semiconductor wafer, for example, and are then used as contact points between an integrated circuit made from the wafer and a suitable chip carrier (in place of wire bonding). The analyzer of the present invention is used to measure the size and thickness of these bumps, in order to verify that they will provide a suitable connection to the chip carrier. To perform the measurement accurately, however, it is necessary that the small spot that is excited by the X-ray source be accurately aligned with one of the bumps. Preferably, directional inhomogeneity of X-ray emission from the bumps is measured so as to provide an indication of misalignment between the spot and the bump, and thus to drive a translation stage so that the spot and the bump are precisely aligned. Alternatively or additionally, the processing unit averages the signals from the different detectors to compensate for any residual misalignment.
In still other preferred embodiments of the present invention, the sample comprises a crystalline substance, such as single-crystal silicon, which generates a diffraction pattern when irradiated by the X-ray source. The diffraction pattern has directional inhomogeneity, whose direction is determined by an orientation angle of the substance. This diffraction pattern can cause anomalies in measurement of X-ray fluorescence by the analyzer. The processing unit detects the inhomogeneous diffraction pattern by detecting differences in the signals that it receives from the different detectors. Most preferably, the signal differences are used to drive a rotation stage so as to align the sample, relative to the detectors, in a manner that minimizes the impact of the diffraction on the fluorescence measurement. Alternatively, the signal differences may be used to determine the crystal orientation.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a method of X-ray analysis, including:
irradiating a spot on a sample with X-rays along an X-ray beam axis;
simultaneously detecting X-rays emitted from the sample, responsive to irradiating the spot, at a plurality of different azimuthal angles relative to the beam axis; and
comparing intensities of the X-rays detected at the different angles in a common energy range in order to determine a property of the sample.
Preferably, irradiating the spot includes irradiating a spot of microscopic size, and comparing the intensities includes determining a geometrical property of a microscopic structure of the sample. Further preferably, simultaneously detecting the X-rays includes detecting X-ray emission using an array of detectors positioned around the spot.
Preferably, comparing the intensities includes detecting an inhomogeneity of the emitted X-rays as a function of azimuth. In a preferred embodiment, detecting the X-rays includes detecting X-rays diffracted from the sample, and detecting the inhomogeneity includes determining an angle of diffraction of the X-rays from a crystalline structure of the sample. Preferably, the method includes introducing a relative rotation between the sample and an array of detectors, responsive to the determined angle, so that the X-rays are diffracted in a desired direction relative to the detectors. Alternatively or additionally, detecting the X-rays further includes detecting fluorescent X-rays emitted by the sample, and comparing the intensities includes using the determined angle to distinguish between the diffracted X-rays and the fluorescent X-rays. Further alternatively or additionally, irradiating the spot includes irradiating a generally symmetrical feature of the sample, and detecting the inhomogeneity includes detecting a misalignment of the spot with the feature.
In a preferred embodiment, detecting the X-rays includes detecting X-rays emitted by a lower feature of the sample, at least a portion of which is covered by an upper feature of the sample so as to block irradiation of the covered portion of the lower feature, and comparing the intensities includes assessing a position of the upper feature relative to the lower feature.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for detecting misregistration of upper and lower layers formed on the surface of a sample, the layers including respective upper and lower features, wherein the upper feature is designed to substantially cover the lower feature, the method including:
irradiating an area of the sample including the upper and lower features with X-rays;
detecting X-rays emitted by the sample in an energy range that is characteristic of the lower feature; and
responsive to an intensity of the detected X-rays, assessing an extent to which the lower feature is not covered by the upper feature.
Preferably, detecting the X-rays emitted by the sample includes detecting fluorescent X-rays emitted by a material that is present in the lower feature. Further preferably, the material present in the lower feature includes a first metallic material, and the upper feature includes a second metallic material. Most preferably, the sample includes a semiconductor wafer, and wherein the upper and lower layers include upper and lower metal layers formed on the wafer.
In a preferred embodiment, the method includes forming a test pattern on the wafer, the test pattern including the upper and lower features, which are designed so that when the upper and lower metal layers are properly registered, the upper feature substantially covers the lower feature. Preferably, detecting the X-rays includes detecting X-rays emitted by the sample at a plurality of different azimuthal angles relative to a beam axis of the irradiating X-rays, and assessing the portion of the lower feature that is not covered includes comparing the intensity of the X-rays emitted at the different angles to determine a direction of misregistration of the upper feature with the lower feature.
Preferably, assessing the extent to which the lower feature is not covered by the upper feature includes determining a measure of the area of the lower feature that is not covered by the upper feature.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for X-ray analysis of a generally symmetrical feature on the surface of a sample, including:
irradiating the feature with a beam of X-rays having a beam diameter at the surface of the sample that is substantially less than a diameter of the feature;
detecting X-rays emitted from the sample, responsive to irradiating the feature, at a plurality of different azimuthal angles relative to an axis of the irradiating beam; and
analyzing a characteristic of the feature responsive to the detected X-rays, using respective intensities of the X-rays detected at the different angles to compensate for a misalignment between the irradiating beam and the feature.
Preferably, detecting the emitted X-rays includes detecting X-ray fluorescence due to an element of the feature, wherein analyzing the characteristic using the respective intensities includes summing the intensities of the X-rays detected at the different angles.
In a preferred embodiment, the feature includes a metal bump formed on the surface of a semiconductor wafer.
Preferably, analyzing the characteristic using the respective intensities includes measuring a difference in the respective intensities of the detected X-rays at opposing azimuths relative to the axis of the irradiating beam. Most preferably, using the respective intensities at the different angles includes relatively shifting the beam and the sample responsive to the measured difference in the intensities, so as to correct the misalignment.
There is further provided, in accordance with a preferred embodiment of the present invention, apparatus for X-ray microanalysis, including:
an X-ray source, adapted to irradiate a spot on a sample with X-rays along an X-ray beam axis;
a plurality of X-ray detectors, arrayed around the spot so as to simultaneously receive X-rays emitted from the sample, responsive to irradiation of the spot, at a plurality of different azimuthal angles relative to the beam axis, and to generate electrical signals responsive to the received X-rays; and
a processing unit,-coupled to receive the electrical signals from the detectors and, responsive to the signals, to compare intensities of the X-rays received at the different angles in a common energy range in order to determine a property of the sample.
Preferably, the X-ray source includes X-ray optics, configured to focus the X-ray beam to a spot of microscopic size on the sample. Most preferably, the X-ray optics include a polycapillary array.
There is moreover provided, in accordance with a preferred embodiment of the present invention, apparatus for detecting misregistration of upper and lower layers formed on the surface of a sample, the layers including respective upper and lower features, wherein the upper feature is designed to substantially cover the lower feature, the apparatus including:
an X-ray source, adapted to irradiate an area of the sample including the upper and lower features with X-rays;
a plurality of X-ray detectors, arrayed around the spot so as to simultaneously receive X-rays emitted from the sample in an energy range that is characteristic of the lower feature, and to generate electrical signals responsive to the received X-rays; and
a processing unit, coupled to receive the electrical signals from the detectors and responsive to the signals, to assess an extent to which the lower feature is not covered by the upper feature.
There is furthermore provided, in accordance with a preferred embodiment of the present invention, a semiconductor wafer having at least an upper and a lower metal layer deposited thereon, the layers including respective upper and lower features defining a test pattern on the wafer, such that the upper feature substantially shields the lower feature from radiation when the upper and lower metal layers are properly registered with one another, but does not shield at least a portion of the lower feature when the layers are not properly registered.
Preferably, the lower feature includes a first metallic material, and wherein the upper feature includes a second metallic material, the first and second materials having substantially different X-ray fluorescence spectra.
There is additionally provided, in accordance with a preferred embodiment of the present invention, apparatus for X-ray analysis of a generally symmetrical feature on the surface of a sample, including:
an X-ray source, adapted to irradiate the feature with a beam of X-rays having a beam diameter at the surface of the sample that is substantially less than a diameter of the feature;
a plurality of X-ray detectors, arrayed around the spot so as to simultaneously receive X-rays emitted from the sample, responsive to irradiation of the spot, at a plurality of different azimuthal angles relative to an axis of the irradiating beam, and to generate electrical signals responsive to the received X-rays; and
a processing unit, coupled to receive the electrical signals from the detectors and, responsive to the signals, to analyze a characteristic of the feature responsive to the detected X-rays, using respective intensities of the X-rays detected at the different angles to compensate for a misalignment between the irradiating beam and the feature.
Preferably, the apparatus includes a translation stage, coupled to be driven by the processing unit so as to shift the sample relative to the detectors, so as to correct the misalignment.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: